This invention relates to combustion devices and more particularly to a controller for insuring optimum combustion conditions.
In most practical combustion devices, fuels are selected for use in a manner that is most compatible with the specific apparatus design, the fuel is conditioned so that it is readily combustible and ambient air is provided as the oxidizer. Both the fuel and air are delivered to a combustion space which is generally enclosed for optimum control of the combustion process and to facilitate the utilization of the heat being produced. The quantities of fuel and air delivered to the combustion enclosure are commonly metered or otherwise controlled to increase combustion efficiency. One important factor in determining such efficiency is the quantity and quality of the oxidizer. Ideally, only a stoichiometric amount of oxygen should be supplied to the reaction zone. Under stoichiometric conditions, the excess gases that are heated to combustion temperature are minimized and maximum combustion chamber temperature may be achieved. The delivery of more air than is theoretically required adds to the total mass required to be heated and results in a temperature reduction. As a practical matter, however, some excess air is usually necessary to guarantee complete oxidation of all fuel components. The quantity of excess air required varies with burner designs but in any case, at least a minimum quantity of excess air is ordinarily necessary to provide complete combustion with an acceptable level of unburned products in the discharge gases.
Most modern combustion equipment includes a fuel-air ratio controller in an attempt to approach optimum combustion additions. However, variations such as the quality of fuel delivered to the combustion zone and ambient conditions will effect the combustion process. For example, the density of air may change quite considerably as a result of changes in barometric pressure, temperature, and humidity. Specifically, at 90.degree. F., 90% relative humidity and a barometric pressure of 29.7 inches of water, there are approximately 0.0154 pounds of oxygen in each cubic foot of air. By comparison, at 50.degree. F., 20% relative humidity and a barometric pressure of 30.5 inches of water, there are about 0.0179 pounds of oxygen in each cubic foot of air. The difference in oxygen in these two examples is 14%. It will be appreciated, therefore, that a boiler which is set to operate optimally on a hot humid day will receive more air than it requires on a day which is cooler and drier. This excess air reduces combustion efficiency by removing additional heat with the flue gas. Further, combustion efficiency is also affected by such variables in the fuel as viscosity, density, carbon-hydrogen ratio, and the water, ash and sulfur contents. Fuel-air metering devices which depend upon fixed mechanical relationships cannot effectively compensate for these variables.
There have been some previous attempts at adjusting the combustion air flow in accordance with measurements of oxygen in the discharged gases. These have usually taken the form of an adjustment in damper position to affect the quantity of air being admitted to the combustion chamber. The adjustment of air flow in this manner has not been wholly satisfactory, however, because of relatively non-uniform response during different portions of the operating cycle, particularly where the boiler employs a rotating or pivotable air damper element. For example, when such boilers are operating in a high fire mode, that is when a relatively large quantity of fuel is being delivered to the furnace and the damper is in a relatively open position, small changes in damper position will not materially affect the amount of air being admitted to the furnace. On the other hand, when the boiler is in a low fire mode wherein only a limited quantity of fuel is being delivered and the air damper is in a relatively closed position, small changes in damper position will affect a relatively large change in the percentage of additional air being delivered. For this reason, in systems which provide air flow adjustment for fuel-air ratio correction, the response to control signaling may be relatively slow at some operating levels and too fast at others, which results in hunting.
An additional shortcoming of the systems in which air flow volume is adjusted in accordance with sensed oxygen levels in discharged gases relates to the difficulty in rendering such systems fail-safe. For example, should the oxygen sensing system fail where damper control is employed, a fail-safe system would tend to return the damper to some neutral position. This position would have to be different for both low and high fire operations and in addition, the degree of correction in each case would also be different. Further, a return of the damper to a neutral position could result in either a lean or rich fuel mixture being delivered to the furnace.